Discovery and naming

William Lassell discovered Ariel and its next companion, Umbriel, on October 24, 1851, and reported his findings after observing these two satellites on three more occasions.[2][3]

The name Ariel, suggested by Sir John Herschel at Lassell's request, is the name of a sylph in Alexander Pope's poem The Rape of the Lock.[5] It is also the name of a sprite and servant to Prospero in William Shakespeare's play The Tempest.[6]

Orbital characteristics

Ariel is in a slightly eccentric orbit around Uranus at an average distance of 191,020 km. Its sidereal month is about 2.52 Earth days. The orbit of Ariel is only slightly inclined from the equator of Uranus but is severely inclined to the ecliptic.

Tittemore[9] suggests that Ariel was once in a 4:1 orbital resonance with Titania. If it was, it is no longer in such a resonance. For this reason, some astronomers suggest that Ariel was much hotter in the past than it is today.[6]

Transits of Uranus

Uranus with Ariel (white) and its shadow (black) at upper right

Ariel is rarely able to transit the surface of Uranus in such a manner as to cast a shadow on it. This is because Ariel, like all the moons of Uranus, is in an orbit with an almost perpendicular inclination to the ecliptic. Such transits usually occur only in years close to Uranian equinoxes. One such transit occurred on July 26, 2006; the Hubble Space Telescope captured that transit on the image at the left. The white dot in the upper-right-hand quadrant of Uranus is Ariel itself; the black dot is its shadow.[11]

Physical characteristics

Surface

The surface of Ariel has relatively few impact craters, and also a system of interconnecting valleys and rifts.[6][4] Some of these valleys bear signs of smoothing by a fluid. Liquid water would not smooth the valley floors in this fashion, and for this reason astronomers suspect that the fluid might have been liquid ammonia or methane. Other valleys and canyons appear to have formed by faulting.

Most of the surface of Ariel appears relatively "young" by uniformitarian standards. In fact, some authorities say that Ariel's surface is the youngest such surface among the major moons of Uranus.[10]

Problems for uniformitarian theories

Ariel poses the same problem for uniformitarian astronomy as do all the other moons of Uranus: its orbit is inclined severely to the ecliptic, though not to Uranus' own equator. How the Uranian system came to have such an inclination has never been explained.

Observation and Exploration

Ariel is visible from Earth with any telescope having an aperture of 30 cm or more.

The one spacecraft to visit Ariel was Voyager 2. Voyager approached to within 127,000 km of Ariel on January 24, 1986, and took several images.[12][13]